1. Field of the Invention
This invention relates to an upconverter, in particular for use in magnetic resonance imaging (MRI).
2. Description of the Prior Art
MRI scanners use a combination of a strong constant magnetic field (B0) from a superconducting magnet which is modified by gradient fields generated by gradient coils, together with a rotating magnetic field (B1) from a radio frequency (RF) antenna to excite nuclear magnetic resonances in the body that generate short term RF signals that are received to build up a tomographic image.
All current-generation MRI scanners employ arrays of local coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The local coils that receive signals from the back of the patient are mounted in the patient table. Local coils that receive signals from the front of the patient are arranged into ‘mats’ that are carefully placed over the patient. Associated with each mat is a flexible cable typically containing one co-axial line for each local coil. The cables interact with the B1 field and with the signals generated from the patient so ‘traps’ (high impedance sections) must be included at regular (typically λ/8) intervals. These add cost and inconvenience to the structure.
In use, the requirement to connect the cables and sterilise them between scanning one patient and the next leads to increased down-time between scans. It is therefore desirable that the cables be eliminated.
Co-pending patent applications GB0915648.0 (corresponding to U.S. Ser. No. 12/726,741 filed Mar. 18, 2010), GB0915653.0 (corresponding to U.S. Ser. No. 12/753,132 filed Apr. 2, 2010) and GB0915655.5 (corresponding to U.S. Ser. No. 12/726,567 filed Mar. 18, 2010) describe a wireless coils implementation that makes use of parametric amplifiers, each connected to one of a number of local coils and each also connected to a microwave antenna, hereinafter referred to as a patient microwave antenna. In addition the bore of the scanner is lined with microwave antennas, hereinafter referred to as bore microwave antennas, tuned to the same frequency as the other antennas. It is arranged that the bore microwave antennas transmit a local oscillator signal that is received by the patient microwave antennas. This signal provides the necessary power and means to make the parametric amplifiers operational to upconvert the local signal to the microwave frequencies. The upconverted signals are radiated from the patient microwave antenna and received at the bore microwave antennas. Receivers, connected to one or more of the bore microwave antennas use the same local oscillator to downconvert the signals back to the original frequency.
Co-pending patent application no. GB0915648.0 (corresponding to U.S. Ser. No. 12/776,741 filed Mar. 18, 2010) describes a hybrid parametric amplifier comprising a low noise amplifier driving a parametric amplifier core as shown in FIG. 4.
The gain and stability of parametric amplifiers is sensitive to the received level of local oscillator (LO), but generally, it is not possible to constrain the variation of local oscillator power to a range less than several decibels.
In another co-pending UK patent application GB0915653.0 (corresponding to U.S. Ser. No. 12/753,132 filed Apr. 2, 2010), a parametric amplifier device comprising a two port parametric amplifier, a DC voltage generator and a DC bias regulator enables the bias voltage in the parametric amplifier to be adjusted in sympathy with the measured received local oscillator voltage in the parametric amplifier in such a way as to detune the parametric amplifier for higher local oscillator voltages such that the gain remained substantially constant. The circuit to do this is potentially complicated and sensitive, particularly when compensation is required under variable drive and load impedance as well as LO level.